RESUMO
A circular economy offers solutions for global sustainability challenges through the transition from the linear take-make-use-dispose economy to a better organisation of resources. However, realising a circular economy has ran into various biophysical constraints. Circular economy implementation is shaped by the Ellen MacArthur Foundation's butterfly diagram that depicts 'biological' and 'technical' flows as separate cycles, subsequently interpreted as organic materials circulating in open loop systems via the environment and inorganic materials circulating in closed loop systems within society. Conversely, in our view, resource flows often contain tightly bound combinations of organic and inorganic materials either due to their natural composition or due to their technical design. Building on this observation, a new diagram is proposed that broadens the scope of the circular economy to cover extractive sectors and the return of materials from anthropogenic use to natural reserves, thereby reshaping the conceptual space within which solutions such as effective zero-waste-residue technologies, business models, and policies can be developed for the optimal management of integrated resources from a whole-system perspective. The diagram offers a realistic outlook on the biophysical limitations of circularity and endeavours to inspire discussion that supports the transition towards a sustainable circular economy.
RESUMO
Biological production of hydrogen is poised to become a significant player in the future energy mix. This review highlights recent advances and bottlenecks in various approaches to biohydrogen processes, often in concert with management of organic wastes or waste CO2 . Some key bottlenecks are highlighted in terms of the overall energy balance of the process and highlighting the need for economic and environmental life cycle analyses with regard also to socio-economic and geographical issues.